The Polish-Soviet War (February 1919 – March 1921) is an armed conflict of Soviet Russia and Soviet Ukraine against the Second Polish Republic and the short-lived Ukrainian People's Republic, four states in post-World War Europe.

The war is the result of conflicting expansionist attempts.

Poland, whose statehood has just been re-established by the Treaty of Versailles following the Partitions of Poland in the late 18th century, seeks to secure territories which she had lost at the time of partitions; the Soviets' aim is to control these same territories, which had been part of Imperial Russia until the turbulent events of the Great War.

Both States claim victory in the war: the Poles claim a successful defense of their state, while the Soviets claim a repulse of the Polish eastward invasion of Ukraine and Belarus, which they view as a part of foreign intervention in the Russian Civil War.The frontiers between Poland and Soviet Russia had not been defined in the Treaty of Versailles and post-war events had created turmoil: the Russian Revolution of 1917; the crumbling of the Russian, German and Austrian empires; the Russian Civil War; the Central Powers' withdrawal from the eastern front; and the attempts of Ukraine and Belarus to establish their independence.

Poland's Chief of State, Józef Piłsudski, feels the time expedient to expand Polish borders as far east as feasible, to be followed by the creation of a Polish-led federation (Międzymorze) of several states in the rest of East-Central Europe as a bulwark against the potential re-emergence of both German and Russian imperialism.

Lenin, meanwhile, sees Poland as the bridge that the Red Army will have to cross in order to assist other communist movements and help conduct other European revolutions.By 1919, the Polish forces have taken control of much of Western Ukraine, with victory in the Polish-Ukrainian War; the West Ukrainian People's Republic has tried unsuccessfully to create a Ukrainian state on territories to which both Poles and the Ukrainians lay claim.

At the same time, the Bolsheviks begin to gain the upper hand in the Russian Civil War and advance westward towards the disputed territories.

By the end of 1919 a clear front has formed.

Border skirmishes escalate into open warfare following Piłsudski's major incursion further east into Ukraine in April 1920.

He is met by a nearly simultaneous and initially very successful Red Army counterattack.

The Soviet operation throws the Polish forces back westward all the way to the Polish capital, Warsaw.

Meanwhile, western fears of Soviet troops arriving at the German frontiers increase the interest of Western powers in the war.

In midsummer, the fall of Warsaw seems certain but in mid-August the tide has turned again as the Polish forces achieve an unexpected and decisive victory at the Battle of Warsaw.

In the wake of the Polish advance eastward, the Soviets sue for peace and the war ends with a ceasefire in October 1920.

A formal peace treaty, the Peace of Riga, is signed on 18 March 1921, dividing the disputed territories between Poland and Soviet Russia.

The war largely determines the Soviet-Polish border for the period between the World Wars.

Much of the territory ceded to Poland in the Treaty of Riga will become part of the Soviet Union after the Second World War, when Poland's eastern borders are redefined by the Allies in close accordance with the British-drawn Curzon Line of 1920.